This invention relates in general to a coolant treatment system for a fuel cell assembly, and deals more particularly with a coolant treatment system for a fuel cell assembly which is in direct communication with an antifreeze coolant solution.
Electrochemical fuel cell assemblies are known for their ability to produce electricity and a subsequent reaction product through the interaction of a fuel being provided to an anode and an oxidant being provided to a cathode, thereby generating an external current flow between these substrates. Such fuel cell assemblies are very useful and sought after due to their high efficiency, as compared to internal combustion fuel systems and the like. Fuel cell assemblies are additionally advantageous due to the environmentally friendly chemical reaction by-products that are produced, such as water. In order to control the temperature within the fuel cell assembly, a water coolant is typically provided to circulate about the fuel cell assembly. The use of reformed fuels within fuel cell assemblies makes them particularly sensitive to possible water contaminants.
Electrochemical fuel cell assemblies typically employ hydrogen as the fuel and oxygen as an oxidant where the reaction by-product is water. Such fuel cell assemblies may employ a membrane consisting of a solid polymer electrolyte, or ion exchange membrane, disposed between the two substrates formed of porous, electrically conductive sheet materialxe2x80x94typically, carbon fiber paper. The ion exchange membrane is also known as a proton exchange membrane (hereinafter PEM), such as sold by DuPont under the trade name NAFION(trademark). Catalyst layers are formed between the membrane and the substrates to promote the desired electrochemical reaction. The combination of the PEM, the two catalyst layers and the substrates are referred to as a membrane electrode assembly.
In operation, hydrogen fuel permeates the porous substrate material of the anode and reacts with the catalyst layer to form hydrogen ions and electrons. The hydrogen ions migrate through the membrane to the cathode and the electrons flow through an external circuit to the cathode. At the cathode, the oxygen-containing gas supply also permeates through the porous substrate material and reacts with the hydrogen ions and the electrons from the anode at the catalyst layer to form the by-product water. Not only does the ion exchange membrane facilitate the migration of these hydrogen ions from the anode to the cathode, but the ion exchange membrane also acts to isolate the hydrogen fuel from the oxygen-containing gas oxidant. The reactions taking place at the anode and cathode catalyst layers are represented by the equations:
Anode reaction: H2xe2x86x922H++2e
Cathode reaction: xc2xdO2+2H++2exe2x86x92H2O
Conventional PEM fuels cells have the ion exchange membrane positioned between two gas-permeable, electrically conductive plates, referred to as the anode and cathode plates. The plates are typically formed from graphite, a graphite-polymer composite, or the like. The plates act as a structural support for the two porous, electrically conductive substrates, as well as serving as current collectors and providing the means for carrying the fuel and oxidant to the anode and cathode, respectively. They are also utilized for carrying away the reactant by-product water during operation of the fuel cell.
When flow channels are formed within these plates for the purposes of feeding either fuel or oxidant to the anode and cathode plates, they are referred to as xe2x80x9cfluid flow field platesxe2x80x9d. These plates may also function as water transfer plates in certain fuel cell configurations. When these plates simply overlay channels formed in the anode and cathode porous material, they are referred to as xe2x80x9cseparator platesxe2x80x9d. Moreover, the plates may have formed therein reactant feed manifolds which are utilized for supplying fuel to the anode flow channels or, alternatively, oxidant to the cathode flow channels. They also have corresponding exhaust manifolds to direct unreacted components of the fuel and oxidant streams, and any water generated as a by-product, from the fuel cell. Alternatively, the manifolds may be external to the fuel cell itself, as shown in commonly owned U.S. Pat. No. 3,994,748, issued to Kunz et al., and incorporated herein by reference in its entirety.
The catalyst layer in a fuel cell assembly is typically a carbon supported platinum or platinum alloy, although other noble metals or noble metal alloys may be utilized. Multiple electrically connected fuel cells consisting of two or more anode plate/membrane electrode assembly/cathode plate combinations are referred to as a xe2x80x9cfuel cell stackxe2x80x9d. A fuel cell stack is typically electrically connected in series.
Recent efforts at producing the fuel for fuel cell assemblies have focused on utilizing a hydrogen rich gas produced from the chemical conversion of hydrocarbon fuels, such as methane, natural gas, gasoline or the like, into hydrogen. This process requires that the hydrogen produced must be efficiently converted to be as pure as possible, thereby ensuring that a minimal amount of carbon monoxide and other undesirable chemical byproducts are produced. This conversion of hydrocarbons is generally accomplished through the use of a steam reformer or an autothermal reformer. Reformed hydrocarbon fuels frequently contain quantities of ammonia, NH3, as well as significant quantities of carbon dioxide, CO2. These gases tend to dissolve and dissociate into the water which is provided to, and created within, the fuel cell assembly. The resultant contaminated water supply may cause the conductivity of the water to increase to a point where shunt current corrosion occurs in the coolant channels and manifold leading to degradation of fuel cell materials, as well as reducing the conductivity of the PEM and thereby reducing the efficiency of the fuel cell assembly as a whole.
As disclosed above, the anode and cathode plates provide coolant channels for the circulation of a water coolant, as well as for the wicking and carrying away of excessive water produced as a by-product of the fuel cell assembly operation. The water so-collected and circulated through a fuel cell assembly is susceptible to water contamination and may therefore damage and impair the operation of the fuel cell assembly as the contaminated water circulates throughout the fuel cell assembly.
It is therefore necessary to provide a system which may protect the fuel cell assembly from water contamination, such as is described in commonly owned U.S. Pat. No. 4,344,850, issued to Grasso, and incorporated herein by reference in its entirety. Grasso""s system for treating the coolant supply of a fuel cell assembly, as illustrated in FIG. 1 of 4,344,850, utilizes a filter and demineralizer for purifying a portion of the coolant supplied to the fuel cell assembly. A deaerator is also utilized to process the condensed water obtained from a humidified cathode exit stream. As discussed in Grasso, the heat exchange occurring between the coolant stream and the body of the fuel cell assembly is accomplished according to commonly assigned U.S. Pat. No. 4,233,369, issued to Breault et al., incorporated herein by reference in its entirety.
It is important to note that Grasso""s coolant system does not provide for the cleansing of the coolant stream as a whole. This is due to the fact that the coolant conduits in Grasso, being fashioned from copper or the like, are not in diffusable communication with the body of the fuel cell assembly and as such, the coolant stream does not receive contamination from, inter alia, the CO2 or NH3 present in the reformed fuel stream. The burden of cleansing the coolant stream in Grasso is therefore born solely by the filter and demineralizer and results in greater wear on these components and hence greater repairs and replacements. Grasso also utilizes two distinct coolant pumps for circulating the coolant.
Another coolant treatment system has been disclosed in commonly assigned Issued U.S. Pat. No. 6,207,308, entitled xe2x80x9cWater Treatment System for a Fuel Cell Assemblyxe2x80x9d, herein incorporated by reference in its entirety. U.S. Pat. No. 6,207,732 utilizes a unique arrangement of demineralizers and degasifiers to cleanse the entire circulating coolant stream while providing for the humidification of an inputted oxidant stream.
In addition to water treatment concerns, the operation of a typical PEM fuel cell may also be adversely affected by extremes in environmental conditions, such as when the operating environment of the PEM fuel cell falls below the freezing point of water. In such circumstances, the volumetric expansion of the water coolant may cause severe damage to the PEM fuel cell. Commonly assigned co-pending U.S. patent application, Ser. No. 09/322,733, entitled xe2x80x9cMethod and Apparatus for Thermal Management of a Fuel Cell Assemblyxe2x80x9d, now U.S. Pat. No. 6,248,462, herein incorporated by reference in its entirety, addresses this additional concern.
U.S. Pat. No. 6,248,462, provides a fuel cell assembly with a cooler plate having channels formed therein for carrying an antifreeze solution, such as a glycol solution or the like, for maintaining the fuel cell assembly above freezing, and alternatively, for quickly raising the fuel cell assembly above freezing during times of cold start-ups. The cooler plate is sealed from communication with the substrate of the fuel cell assembly to protect against contamination of the substrate and catalyst materials and subsequent failing of the fuel cell assembly as a whole. The addition of a cooler plate in a fuel cell assembly creates an associated increase in the weight and volume of the fuel cell assembly which is only exacerbated when a plurality of planar fuel cell assemblies are joined together to form a fuel cell stack. This increase in both weight and volume is especially troublesome and undesirable in applications involving fuel cell powered vehicles and the like.
Accordingly, commonly assigned co-pending U.S. patent application Ser. No. 09/359,475, entitled xe2x80x9cDirect Antifreeze Cooled Fuel Cellxe2x80x9d, now U.S. Pat. No. 6,316,135, herein incorporated by reference in its entirety, discloses a coolant system for use with a fuel cell assembly whereby an antifreeze solution is in fluid communication with the fuel cell assembly, but is kept from contaminating the electrolyte and catalyst through a judicious balance of pressures within the fuel cell assembly and the wetproofing of certain constituent elements of the fuel cell assembly.
With the forgoing problems and concerns in mind, it is the general object of the present invention to provide a fuel cell assembly with a coolant treatment system which overcomes the above-described drawbacks. Accordingly, an integrated coolant treatment system is proposed having an antifreeze cooling solution which is not isolated from the components of the fuel cell assembly, and which also minimizes the accumulation of pure water within the fuel cell assembly.
It is an object of the present invention to provide a coolant treatment system for a fuel cell assembly.
It is another object of the present invention to reduce the levels of contaminants within the coolant circulating throughout a fuel cell assembly.
It is another object of the present invention to provide a coolant treatment system which also humidifies the oxidant flows to the cathode of a fuel cell assembly.
It is another object of the present invention to reduce the possibility of contaminating gas build-up within the coolant system.
It is another object of the present invention to protect a fuel cell assembly from the debilitating effects of freezing temperatures.
It is another object of the present invention to reduce the weight and volume of a fuel cell assembly which is protected against freezing temperatures.
It is another object of the present invention to provide a fuel cell assembly with an antifreeze coolant solution which is in direct fluid communication with the fuel cell assembly.
According to one embodiment of the present invention, a coolant treatment system for a fuel cell power plant has a plurality of electrochemical fuel cell assemblies in electrical connection with each other, the fuel cell assemblies each having an electrolyte, an anode, a cathode. The anode and the cathode are each adapted to support anode and cathode water transport plates through which a fuel and an oxidant are fed to the anode and the cathode, respectively. In addition, one of the anode and the cathode water transport plates is adapted to support a coolant channel through which an antifreeze solution is circulated. The antifreeze solution is in fluid communication with one of said anode and cathode water transport plates.
An oxidant source is utilized to provide the fuel cell power plant with the oxidant, while a coolant conduit exhausts the antifreeze solution from the fuel cell power plant.
In operation, a degasifying apparatus treats the antifreeze solution from the coolant conduit together with the oxidant from the oxidant source by removing contaminants from the antifreeze solution and by humidifying the oxidant. The degasifying apparatus subsequently provides the fuel cell power plant with the humidified oxidant.
These and other objectives of the present invention, and their preferred embodiments, shall become clear by consideration of the specification, claims and drawings taken as a whole.